HIGH RESOLUTION CRYSTAL STRUCTURE OF A GUANINE TETRAPLEX. Ben Luisi, Kathryn Phillips, Zbyszek Dauter*, Alaistair Murchie** and David Lilley** Department of Biochemistry, Cambridge Univ., Tennis Court Road, CB2 1QW, U.K. *Department of Chemistry, York Univ. Heslington Y01 5DD, U.K. **Department of Biochemistry, Dundee Univ., Dundee DD1 4HN, U.K

We report the crystal structure of a DNA molecule with four, parallel strands at 0.95 Å resolution. This tetraplex DNA is formed by d(TGGGGT), and sixteen strands occupy the asymmetric unit. The structure is stabilized by tetrads of hydrogen-bonding guanines and by coordination of sodium ions between successive tetrad planes. The high resolution structure permits a detailed analysis of the hydration structure in and around the helical grooves and of the interaction of ions with the phosphate groups. The sugars have a well defined puckering geometry and distinctive base-stacking interactions are observed. Four-stranded DNA structures have been implicated in the organization of telomeres and other large assemblies, and these functions will be discussed.